1. Field
The invention relates to a method and a device for operating a flow meter, in particular magnetically inductive flow meters but also capacitive meters, as well as those which comprise electrodes which can be used in order to feed a signal into the fluid to be measured, according to the preambles of patent claims 1 and 10.
2. Related Art
The most popular flow meters are ones which are operated magnetically inductively. In this case a magnetic field is induced via a defined measurement tube, and a signal generated by the magnetic field is measured by at least one pair of electrodes, which have a junction with the fluid.
Methods and devices of this type are nowadays often used with diagnostic means for recording the status of the device on the one hand, as well as for recording perturbations in the flow of the measurement medium on the other hand. To this end, the actual measurement signal determining the flow rate is used only secondarily. It is primarily the arrangement of electrodes, which are in contact with the measurement medium, that is used in order to obtain significant measurement values electrically. When feeding signals via the electrodes, limit values are set technically in respect of voltage and current, which reliably prevent electrolysis processes from being induced at the electrodes in the measurement medium.
Besides the functional perturbations of the meter per se, however, otherwise uniform flows of the measurement media may also enter perturbed states, for example owing to the cavity effect in which turbulent flows past tube contours experience such negative pressures that gas bubbles, which then also have to pass through the flow meter device, nevertheless suddenly occur in an otherwise gas bubble-free liquid medium. Furthermore, other materials such as solids, contaminants or the like in the measurement medium may also cause such perturbations that, although a sum flow rate is in fact thereby measured, similarly as in the case of gas bubbles this does not however correspond exclusively to the flow rate of the actually desired measurement medium. In particular, recording this status proves particularly difficult.
Another problem occurs because in many cases the liquid media generate deposits on the electrodes and/or on the insulating region of the measurement tube, the so-called liner, after a corresponding operating time. These deposits also vitiate the measurement result and therefore the displayed flow rate relative to the true flow rate.
For instance, DE 102 43 748 and DE 101 18 002 disclose an electromagnetic flow meter in which impedances between one or more electrodes and a grounding point are measured. With these, varying resistance values can be compared with empirical values and where applicable deposit formation can be inferred therefrom.
Significant recording of gas bubbles in the flow, for example, is not however provided in this case. Merely growing resistances, particularly at the junction between the electrode and the measurement medium, are diagnosed significantly as deposit-forming.